Naturally Rehearsing Passwords

1. Naturally Rehearsing Passwords Jeremiah Blocki NSF TRUST October 2013 Anupam Datta Manuel Blum

2. Password Management … Competing Goals:

3. A Challenging Problem Use numbers and letters Use special symbols Don’t Reuse Passwords Don’t use words/names Not too short Don’t Write it Down Use mix of lower/upper case letters Change your passwords every 90 days Traditional Security Advice

4. Experiment #0 Memorize the following string L~;z&K5De

5. Memory Experiment 1

6. Memory Experiment 2

7. Outline Introduction and Experiments Example Password Management Schemes Quantifying Usability Quantifying Security Our Password Management Scheme

8. Password Management … Competing Goals:

9. Scheme 1: Reuse Strong Password • Pick four random words w1,w2,w3,w4

10. Scheme 2: Strong Random Independent Four Independent Random Words per Account

11. Questions • How can we evaluate password management strategies? • Quantify Usability • Quantify Security • Can we design password management schemes which balance security and usability considerations?

12. Outline • Introduction and Experiments • Example Password Management Schemes • Quantifying Usability • Human Memory • Rehearsal Requirement • Visitation Schedule • Quantifying Security • Our Password Management Scheme

13. Human Memory is Semantic Memorize: nbccbsabc Memorize: tkqizrlwp 3 Chunks vs. 9 Chunks! Usability Goal: Minimize Number of Chunks Source: The magical number seven, plus or minus two [Miller, 56]

14. Human Memory is Associative ?

15. Cues • Cue: context when a memory is stored • Surrounding Environment • Sounds • Visual Surroundings • Web Site • …. • As time passes we forget some of this context…

16. Human Memory is Lossy • Rehearse or Forget! • How much work? • Quantify Usability • Rehearsal Assumption pamazon ???? pgoogle

17. Quantifying Usability • Human Memory is Lossy • Rehearse or Forget! • How much work does this take? • Rehearsal Assumptions • Visitation Schedule • Natural Rehearsal for frequently visited accounts

18. Rehearsal Requirement Expanding Rehearsal Assumption: user maintains cue-association pair by rehearsing during each interval [si, si+1]. Day: 1 2 4 5 8 Visit Amazon: Natural Rehearsal Google Xt: extra rehearsals to maintain all passwords for t days.

19. Rehearsal Requirement Day: 1 2 4 5 8 Xt: extra rehearsals to maintain all passwords for t days.

20. Visitation Schedule Poisson Process with parameter 𝞴 Cueshared by Amazon and Google +𝞴 t1 t2 t2

21. Visitation Schedule Poisson Process with parameter  Amazon Google Day: 2 4 5 8 Number of accounts visited with frequency 

22. Usability Results E[X365]: Extra Rehearsals to maintain all passwords over the first year. Usable Unusable

23. Valuable Resources Protected by Passwords

24. Outline • Introduction and Experiments • Example Password Management Schemes • Quantifying Usability • Quantifying Security • Background • Failed Ideas • Our Approach: Security as a Game • Our Password Management Scheme

25. Security (what could go wrong?) Three Types of Attacks Danger

26. Online Attack 123456 password 123456 Guess Limit: k-strikes policy

27. Offline Dictionary Attack jblocki, 123456 SHA1(12345689d978034a3f6)=85e23cfe0021f584e3db87aa72630a9a2345c062 +

28. Plaintext Recovery Attack pwd PayPaul.com pwd

29. Snowball Effect pwd pwd PayPaul.com + Source: CERT Incident Note IN-98.03: Password Cracking Activity

30. Our Security Approach • Dangerous World Assumption • Not enough to defend against existing adversaries • Adversary can adapt after learning the user’s new password management strategy • Provide guarantees even when things go wrong • Offline attacks should fail with high probability • Limit damage of a successful phishing attack

31. Security as a Game p1 p2 p3 p4 p5 Sha1(p4) PayPaul.com p5 q$1,000,000 guesses +

32. The Adversary’s Game pwd Sha1(pwd) • Adversary can compromise at most r sites (phishing). • Adversary can execute offline attacks against at most h additional sites • Resource Constraints => at most q guesses • Adversary wins if he can compromise any new sites.

33. (q,,m,s,r,h)-Security For any adversary Adv s = # online guesses q = # offline guesses r = # h = # Phishing Attack Accounts m = # of accounts Offline Attack Accounts

34. Example: (q,,m,3,1,1)-Security h=1 PayPaul.com + q guesses r=1

35. Security Results (q$1,000,000,,m,3,r,h)-security Usable + Insecure Unusable + Secure

36. Outline Introduction and Experiments Example Password Management Schemes Quantifying Usability Quantifying Security Our Password Management Scheme

37. Our Approach Private Public Cue • Object: bike Action: kicking • Object: penguin

38. Login … Pwd • Kic+Pen + Tor + Lio+ ...

39. Login … Pwd Kic+Pen + ….

40. Sharing Cues • Usability Advantages • Fewer stories to remember! • More Natural Rehearsals! • Security? Day: 1 2 4 5 8

41. (n,l,)-Sharing Set Family Definition: A (n,l,)-Sharing Set Family of size m is a family of sets {S1,…,Sm} with the following properties n n

42. (n,l,)-Sharing Set Family m – number of passwords {S1,…,Sm}. n – total #PAO stories l – #PAO stories for each site – max intersection – PAO stories for account i. n n

43. Security Results (q$1,000,000,,m,3,r,h)-security

44. Sharing Cues Thm: There is a (43,4,1)-Sharing Set Family of size m=90, and a (9,4,3)-Sharing Set Family of size 126 • Proof? • Chinese Remainder Theorem! • Notice that 43 = 9+10+11+13 where 9, 10, 11, 13 are pair wise coprime. • Ai uses cues: {i mod 9, i mod 10, i mod 11, i mod 13}

45. Chinese Remainder Theorem By the Chinese Remainder Theorem there is a unique number x s.t 1) 2) 3) Hence, for accounts Aiand Ajcannot use the same red cue and blue cue.

46. Example(Account #80)

47. Example(Account #80) Public Cue for Account 80

48. Usability Results E[X365]: Extra Rehearsals to maintain all passwords over the first year.

49. Security Results (q$1,000,000,,m,3,r,h)-security Usable + Insecure Unusable + Secure Usable + Secure Usable + Secure

50. Experiment #0 Can anybody remember the 10 character password? L~;z&K5De